For a liquid crystal display (LCD), the response time indicates the speed when each pixel in the LCD responses to an input signal, i.e., the response time indicates the time consumed during the pixel transforming from black to white or from white to black, which is an important factor for determining the characteristic of the LCD. The response time directly affects the quality of the pictures, especially the motion pictures, displayed by the LCD. Thus, it is crucial to measure the response time for determining the characteristic of the LCD. Generally, for most of the LCDs, the response time is defined by the criteria 305-1 managed by the Video Electronics Standards Association (VESA). Specifically, two different gray levels (regarded as the gray level A and the gray level B below) are preset first. The change of the gray levels is corresponding to the change of the luminance of the LCD. The response time is referred to as the transition time during which the luminance is converting from an initial value to 90% of an objective value. Then, a rise time (Tr) and a fall time (Tf) are measured. During the rise time (Tr), the luminance is rising from 10% to 90% and the images displayed by the LCD are changing from the grey level A to the grey level B. During the fall time (Tf), the luminance is falling from 90% to 10% and the images displayed by the LCD are changing from the grey level B to the grey level A. The response time of the LCD is defined as the sum of the rise time (Tr) and the fall time (Tf).
At present, the general method for measuring the response time comprises: collecting motion optic signals on the LCD via an actual luminance measuring device (an optical sensor); drawing a luminance-time curve (a response time curve); obtaining the time period of the rise edge (from 10% to 90%) and the time period of the fall edge (from 90% to 10%) from the curve, which are regarded as the rise time and the fall time of the response time respectively.
Although the response time is defined such definitely, those skilled in the art often use different actual luminance measuring devices to measure the response time. Actual luminance measuring devices with different types and different models, such as Charge-Coupled Device (CCD), Photo Diode, Photo-multiplier Tube (PMT) and the like, may be used in the system for measuring the response time. Since different actual luminance measuring devices have different luminance induction linearity, the response time measured by different actual luminance measuring devices will be different from each other, which in turn cause an error for the measurement result. FIG. 4 shows two normalized response time curve ranging from the 192 gray level to the 225 gray level, namely the luminance-changing-over-time curve, measured by OPT101 and S9219 photodiode respectively. The difference between the response time measured by the OPT101 and the response time measured by the S9219 is 8 ms, as shown in the circle label in FIG. 4, which is a big error in practice.